A urea water supply system includes: a first supply valve for supplying urea water; a second supply valve for supplying urea water; a supply passage for connecting a urea water tank and the first and the second supply valves; and an electronic control unit (ECU). The supply passage branches to have a first supply passage extending from a branch point to the first supply valve and a second supply passage extending from the branch point to the second supply valve, and a volume of the first supply passage is greater than a volume of the second supply passage. The ECU opens the first supply valve while keeping the second supply valve closed, for starting filling of the supply passage with urea water, and thereafter determines completion of filling of urea water into the first supply passage.